The present invention relates to a hydraulic shock absorber suitable for use in a suspension system of a vehicle, for example, an automobile.
In general, a cylinder type hydraulic shock absorber attached to a suspension system of a vehicle, e.g. an automobile, includes a cylinder having a hydraulic fluid sealed therein. A piston is slidably fitted in the cylinder. The piston is connected with a piston rod to form a piston assembly. The piston assembly is provided with damping force generating mechanisms each including a hydraulic fluid passage, an orifice, a disk valve, etc. The hydraulic fluid is induced to flow through the hydraulic fluid passage by sliding movement of the piston in the cylinder caused by the stroke of the piston rod. The flow of the hydraulic fluid is controlled by the orifice and the disk valve to generate damping force. In a low speed region of the piston speed, damping force is generated by the orifice. In a high piston speed region, the disk valve deflects to open, thereby preventing an excessive rise in damping force.
In the above-described conventional hydraulic shock absorber, however, the damping force generated in the low piston speed region depends on the flow path area of the orifice. In the high piston speed region, the damping force depends on the preset valve opening pressure of the disk valve. Accordingly, the conventional hydraulic shock absorber suffers from a low degree of freedom for setting damping force characteristics.
Under these circumstances, a hydraulic shock absorber designed to increase the degree of freedom for setting damping force characteristics has been proposed, as disclosed, for example, in Japanese Patent Application Unexamined Publication (KOKAI) No. Hei 2-278026. The hydraulic shock absorber has at the back of the disk valve a back-pressure chamber, a valve (second disk valve) for controlling a pressure introduced into the back-pressure chamber, and a relief valve that opens when the pressure in the back-pressure chamber reaches a predetermined pressure to relieve the pressure. A part of hydraulic fluid is introduced into the back-pressure chamber through an orifice passage to allow the pressure in the back-pressure chamber to act on the disk valve in the direction for closing the valve. The back pressure acting on the disk valve is adjusted by the valve (second disk valve) and the relief valve. More specifically, in the low piston speed region, the valve (second disk valve) is closed. Therefore, small damping force is generated. When the piston speed rises, the valve (second disk valve) opens to generate large damping force. When the piston speed rises further, the relief valve opens to generate damping force smaller than the above-described large damping force. Thus, it is possible to increase the degree of freedom for setting damping force characteristics.
However, the above-described conventional hydraulic shock absorber provided with a back-pressure chamber has the following problem. Because a valve (second disk valve) for controlling the pressure to be introduced into the back-pressure chamber is provided in addition to the disk valve that receives the pressure in the back-pressure chamber, the damping force generating mechanism becomes complicated in structure and long in axial length, resulting in an increased overall size.
The present invention was made in view of the above-described circumstances. Accordingly, an object of the present invention is to provide a hydraulic shock absorber that is capable of increasing the degree of freedom for setting damping force characteristics to obtain appropriate damping force characteristics, and that has a damping force generating mechanism simplified in structure to allow a reduction in overall size.